Allegro A3977, Idle noise Help

Matilda · Aug 13, 2006

Hello All,
I'm designing a CNC system using an Allegro 3977.
All is running well.
When the motors are enabled, and I'm sitting still, the motors have quite a high pitched whine to them.
My sense resistors are 0.2ohm. Ohmite current sense resistors.
My steppers are Applied Motion Product Steppers. HT23.
I believe they are 3.6ohm, rated at 1.4Amps.

I've been messing around with the Vref, the PFD.
And not getting anywhere!
I can adjust the PFD and hear the pitched whine changing frequency, but the volume of this pitch is too loud.

My RC resistors are 30K, with the 0.1UF caps.

Let me know!

hjames · Aug 13, 2006

What a coincidence - I just setup my CNC with the same chip about a month ago. I do notice a ~10 - 15KHz or so high pitched whine, but I never bothered looking into it. I used to work with a relatively high end desktop CNC machine a couple years back, and remember a similar sound so I never bothered to look into it.

I don't have the particular timing values, but I'll dig them up later.

James

Matilda · Aug 14, 2006

Is this whining detrimental to the motors, or the chip?

phalanx · Aug 14, 2006

The whine is mostly just an annoyance and shouldn't be detrimental to the motor. I have run into similar issues while using the A3984 and found a few ways to reduce the problem.

The first thing is to decrease the fixed off time to the minimum possible while still meeting your current ripple requirements. This could increase the power dissipated by the chip itself due to an increased switching rate.

The other thing I did was increase the voltage to the motors. Increasing voltage decreases the time it takes for the current to ramp up through the inductance of the motor. This effectively increases your switching frequency and if you raise it enough, it will become inaudible.

Matilda · Aug 14, 2006

Thanks Phalanx.
I tried cranking up the voltage from 24V to 35V, while keeping the logic voltage at 5V.

I was using a variable power supply. As I was cranking the supply up from 24V to 35V, I could hear the frequency changing, I believe it was getting higher. But over all, it did not do much for me.

"Decrease the Off time"
Is that done by the resistor//Cap. I believe Allegro calls it as the RC1/RC2 inputs?
Allegro specs a 30Kohm & a 1nF cap.
I tried lowering this to 20Kohm... Maybe I should go the other way?
Like towards 40K?

I saw a demo schematic by a company called "E-LAB".
They have 36Kohm//1nF for the RC# inputs.

It's driving me nuts here.
All 4 motors are just humming away....LOUD.

philba · Aug 14, 2006

Basically, you are hearing the pwm frequency. (ok, pseudo-PWM frequency). Decreasing off time Toff increases the frequency and can push it above the hearing threshold. so what if it annoys the dogs?

Increasing the voltage slightly decreases the time it takes for the current to reach the Itrip level. you still have a fixed off time. thus the slight effect on the frequency of the noise.

The resistor and cap don't form an RC oscillator. Toff = RC, according to Allegro so decrease it. the cap is also used for the comparator blanking (to prevent false shut off due to current bounce) so I wouldn't change that.

I would not run those chips at 35V. While it is at the upper limit of the spec, I've heard of people blowing the chip at that voltage. best to stay around 30V.

Matilda · Aug 14, 2006

So you think I should try lowering those two resistors?
The two found at Pins 10 & 15 of the 44PLCC version?

I should put in some 10Kohm and see what it does.

Thanks for all of your help!!

phalanx · Aug 14, 2006

philba said:
I would not run those chips at 35V. While it is at the upper limit of the spec, I've heard of people blowing the chip at that voltage. best to stay around 30V.

I've heard of that happening as well but have never seen it first hand. It seems to involve running motors at 35V with a current at or near the limit of the chip. I think it has something to do with inductive kickback and the associated heat being dissipated by the chip. If you are running at high voltage and current and have a relatively large winding inductance, I would suggest putting schottkys around the motor.

Matilda...

If you are not using external diodes, make sure that synchronous rectification is enabled on your chip. Otherwise make sure you have schottkys around the motor.

Matilda · Aug 14, 2006

I have attached a little photo of my board, showing only one of the 4 channels.
I'm using these isolated 5v:5v regulators, and I made the footprint wrong in orcad. Ignore that mistake! Corrected for REV;B board

Anyway, I added the 8 "Ultra Fast Avalanche Sinterglass Diodes" to the output/motor leads.
I also have the SR line pulled LOW(Active Mode-Sync Rec Occurs).

I also kept the grounds isolated.
As in,
To the A3977 I have 5V & 24Volts going to the chip.
Pins 44,1,2 22,23,24 33,34,35 are connected to the 24V supply ground.
Pins 11,12,13 are connected to the 5V supply.
With the chip off the board, these two ground paths are isolated.
The chip connects them internally.

That was a tip from Allegro. It really helped minimize noise seen on the 5v and 24V supply lines while the motor was running.

philba · Aug 14, 2006

the external diodes need to be faster than the internal ones or they are useless. the idea is to have the chip dissipate less energy. I'd hope schottkys would work.

everything I've seen says that allegro was a bit optimistic in spec'ing the upper limits on that chip.

I've not mucked with Toff but since the app note has the formula Toff = RC, I'd say that decreasing R should reduce off time.

Matilda · Aug 14, 2006

I changed out those RC resistors.
I put in some 10Kohm resistors. Much quieter

when sitting still.

And as a double bonus, the stepper is just cranking out the REVS!

Before I changed out the resistors, I couldn't get the function generator (STEP signal) to go past 18Khz.
Now it will reach 62Khz. Awesome!

I'm running the motor continously at various speeds between 10Khz & 60Khz. The chip isn't getting warm at all.
A little more tweaking and I may just have it!

Thanks All,

If you have more thoughts let me know.
And what ever else I find out, I will let you all know.

Aphromann · Aug 15, 2006

Use egg cartons as a sound buffer, itll make it quiter if the sound is not harming the robotics.

Matilda · Aug 15, 2006

I'm driving plasma torches.
Many sparks!

Sparks and egg cartons are not allowed to play together

philba · Aug 15, 2006

lol, the though of sound dampening in my workshop makes me chuckle. nothing like something else to snag the dust.

by the way, the whine won't harm the motors at all. just your sanity. how many IPS (CMPS?) are you getting with your new found power. sounds pretty good.

Matilda · Aug 16, 2006

What is "IPS (CMPS?)".

I'm in the electronics/controls business, but I'm very new to the stepper motor world.

hjames · Aug 16, 2006

Inches Per Second - spinning a stepper is fun, but rapid'ing the CNC world's "mine's bigger than yours" comparison.

Matilda · Aug 16, 2006

IPS. Wouldn't that be based on the diameter of the gear at the end of the stepper shaft?
Or is IPS based on a standard gear size?

I have not tested this IPS, but from math.....does the following make sense?
(Driver at 1/8 step. Motor 200s/r or 1.8deg. Gear Perimeter 2.25")
So each full turn of the shaft, the motor travels 2.25".

With a STEP signal of 60Khz...
60Khz/(200 steps/rev) = 300 Turns in 1 sec...

So if my stepper is turning 300 times in 1 sec....
And my travel distance in 1 rev is 2.25"
Thats 675"... Somethings wrong?
That's 56'

I'll have to try it out?

Again with the IPS.
Is that based on a standard step signal, or gear size?

I just feel there are too many variables to compare IPS.

hjames · Aug 16, 2006

It's assumed that the stepper(or servo) motors are already installed in the CNC setup, and the measurement is relative to the machining head. At some point steppers will start to torque-out(i.e. miss steps) and it can't be used above that point. It's not supposed to be a serious question...

philba · Aug 16, 2006

inches per second, cm per second.

Matilda · Aug 16, 2006

Oh OK.
Hey I'm newbie....just learning the ropes.
The gantry that is driven by these steppers is not allowed to run past 5"/sec. That is a spec'd concern from my customer.
I know it's going 5"/sec.

(Looks slow to me)

End user is happy & he likes the smoothness.

Thats all that counts I guess.

Thanks guys

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